Looking into the chemistry and sensory effects of bushfire smoke on red wines

Author: Engela Duvenage

Published: 23/09/2019

As has been very evident from recent news from around the world, wildfires are becoming an increasingly widespread and devastating influence on our landscapes, ecosystems and economies. The effect of smoke on agricultural crops is thus receiving much more attention. The possible influence that compounds involved in a phenomenon known as 'smoke taint' in wine originating from grape exposure to smoke from bushfires was tackled in a recent article in the South African Journal of Enology and Viticulture, and also in a popular article written for the website Sciencetrends.com. Links are included below.

As part of this work, researchers in the Department of Viticulture and Oenology in the Faculty of AgriSciences at Stellenbosch University, led by Dr Marianne McKay and Dr Astrid Buica tested twelve commercial red wines that were potentially smoke tainted based on the provenance (origin and harvest time) of the wines. Earlier this year, Dr McKay received her PhD in the Faculty of AgriSciences on the topic.

Wine aroma

Wine aroma is arguably the most important intrinsic factor used to judge wine quality. The perception of wine aroma is the result of a complex interplay of factors including the composition of volatile compounds in the wine, the perceptual interactions between the volatiles, the physical and chemical effects of non-volatile components of the wine matrix, and the ability and experience of the person perceiving the aroma.

Aroma compounds (not all of them pleasant) can enter wine from different sources during the production process. For example, herbaceous-smelling methoxypyrazines are part of primary aroma, meaning that they are naturally present in some grape varieties, and are extracted from the grapes during winemaking. Other compounds including esters are released by yeast during fermentation and are considered part of the secondary (processing-related) aroma of the wine. Wine can also have tertiary aromas that develop later, for example, those deriving from oxidation compounds that form during long-term storage.

Problematic aromas

Wines can also exhibit unpleasant aromas that fall into the spectrum of off-odours or taints. When wine is made from smoke-affected grapes, smoke volatiles are transferred from the grapes during the winemaking process, and manifest in the wine as an unpleasant burnt, ashy aroma (smoke taint). Some of the main culprits causing this off-odour are volatile phenols (VPs). This group of compounds can also be derived from other sources including toasted oak barrels, microbial fermentations (specifically Brettanomyces spoilage), but they have become mostly associated with off-odours in wine made from smoke-affected grapes.

The usual way to deal with a problematic aroma compound is to quantify it by chemical/analytical means, and then compare the measured value to the compound's Odour Detection Threshold (ODT, the lowest concentration of a compound that is perceivable by smell). If the measured compound is present at concentrations above its ODT, it is considered to have an impact on the smell of the wine, and we should be able to smell it.

Like any other aroma compound, VPs can be quantified by analytical means. Previous research has established that, at higher levels, certain VPs contribute to a continuum of smoke taint related off-flavours including “burnt", “bretty", “smoky" and “ashy". At subthreshold levels, VPs are generally accepted as being benign to wine aroma.

The influence of smoke

As previously noted, increasing importance is being placed on understanding the effects of smoke on agricultural crops, and there are numerous articles available on the effects of smoke on grapes and wine. Most of these reports are the result of experimental conditions looking into replicating field conditions in a reproducible manner in small batches, including micro-vinifications. In contrast, research into the composition of commercial wines affected by naturally-occurring (i.e. not experimentally induced) smoke events is still rare. As such, we decided to investigate the impact of bushfires on the chemical (VP concentrations) and sensory profile of commercially produced red wines.

How was the study done?

Twelve wines were submitted by the industry as potentially smoke-tainted since the grapes had been exposed to bush fires close to harvest time. The Stellenbosch University research team investigated historical data concerning fire events in the regions from which the wines originated, and during the relevant vintages, using archives from fire-tracking satellite information systems to confirm whether fires had actually occurred.

The wines were chemically screened for a broad range of VPs using a specially developed GC-MS method. The wines were also sensorially characterized using Descriptive Analysis by a panel highly trained in smoke-taint evaluation. The results were compared statistically to see if the chemistry results did actually support the findings from the sensory evaluation. If this was the case, a relatively simple chemical screening of VPs would be enough to establish if a wine is smoke tainted or not.

Findings

The GC-MS results indicated the wines contained a variety of VPs and a wide range of VP concentrations. On the sensory side, it was notable that out of twelve wines, the four that were described with the most negative attributes, at significantly higher levels than the others, were all from regions that had experienced severe fire events prior to harvest. Certain attributes (“smoky," “ashtray") in some of the wines could be attributed simply to levels of specific or combinations of VPs at concentrations higher than their respective ODT.

Guaiacol (one of the compounds linked to smoke taint) was present in the majority of samples at or above ODT. As the wines had been submitted by industry for suspected smoke taint, this result was not surprising.

However, there were some unexpected findings. With regard to guaiacol, it was found that this compound did not cause the perception of “smoke" in any of the wines unless it was in combination with other phenols. Even more surprising, combinations of compounds (for example, cresols and xylenols) at subthreshold levels led to unexpected sensory effects (“earthy/dusty," “chemical," and “tar/burnt rubber").

Using the combined dataset (chemistry and sensory), the research team started to see that there were (sometimes unexpected) links between VPs and specific off-odours. Some of these links were previously demonstrated, others we could not predict based on existing knowledge. Wines with very low (sub-threshold) levels of VPs showed fruity and sweet-associated characteristics, and those with supra-threshold (above threshold) levels showed negative attributes. In some cases, sensory effects (“earthy/dusty/potato skin", “mouldy/musty" and “cooked vegetables") could not be attributed to a certain concentration of VPs based on their ODT, but may have been due to combinations of volatile phenols at subthreshold levels, possibly influenced by the presence of other compounds.

The idea of interactions with masking and synergistic effects is not conceptually new, and perceptual interaction phenomena between aroma compounds in red wines represent an important source of complexity. It has also been already demonstrated in studies on other compounds (thiols, esters, methoxypyrazines, terpenes, etc.) that sensory results do not always correlate well with predictions that are based on the chemistry of the solution.

However, the SU team's findings did highlight a number of issues. The aspect of “matrix effect" was significant in the context of our work, as the commercial wines were diverse, i.e. different chemical matrices as dictated by cultivar-related grape genetics, ripeness levels, winemaking procedures, and bottle storage. A significant gap exists in the literature with regards to matrix effects on VPs, specifically the effect of the VP composition on different cultivar aroma profiles.

Odour thresholds in any matrix other than the study matrix may be irrelevant. The literature review also revealed that studies in wine can use inappropriate ODTs for work carried out in a new wine matrix, and matrix effects are often ignored. More to the point, many VPs (as well as other compounds) have not yet been characterized in any wine matrix or assigned threshold values in any formal sensory study.

The study also highlighted the fact that that aroma compounds interact even at subthreshold levels and cause aromatic changes to wine, unpredictable from the effects of a single compound, and certainly not predictable from the many peri-threshold studies on aroma compounds. So even if the ODT is acc

urately determined in the wine matrix of choice, it is still possible to find sensory effects when the compound is present at concentrations below threshold, especially in the presence of certain other compounds.

This study emphasized the importance of understanding the effects of problematic compounds like VPs on wine and of increasing awareness of the interactions and synergies between them even at levels where they are usually considered benign. Crucially, low levels of VPs may cause or exaggerate negative odour attributes in red wines, which has implications for winemaking with smoke-affected grapes. This new knowledge should prove useful to wine (and other agricultural) industries world-wide that are increasingly being affected by smoke taint. Ultimately, the aim is to produce wines that satisfy the rigorous demands of the global market despite the escalation of wildfires globally.